Required Time of Arrival (RTA) guidance is a widely available technology employed in commercial aircraft. FIG. 1 illustrates a feedback loop that is used to control aircraft speed during cruise and descent.
Guidance of the aircraft involves specification of a Required Time of Arrival at a target waypoint of the flight plan (block 100 of FIG. 1). The aircraft flight management system then periodically predicts an Estimated Time of Arrival (ETA) at the specified target waypoint based upon various measured (e.g., sensed) and predicted variables such as current wind speed/ground speed, weather forecast etc., (also block 100 of FIG. 1).
At block 110 of FIG. 1, a deviation between the RTA and the ETA is determined and, if it is below a chosen threshold DT, then no action is taken. If however the deviation between RTA and ETA exceeds the chosen threshold DT, then at block 120, the aircraft guidance modifies the cruise/descent speeds as part of a new planned trajectory to meet the RTA. At block 130, the planned trajectory (e.g., flight plan) with the modified speeds is executed. Such modifications of planned trajectory speeds take place several times during cruise and descent before arrival at the target waypoint.
The threshold DT is variable in the sense that it is intentionally larger the further away from the waypoint the aircraft is. For example, the threshold time difference between RTA and ETA (below which no adjustments are mandated) may be 1 minute when the aircraft is 200 nautical miles (NM) from the waypoint. This is because the degree of uncertainty in the ETA increases with distance from the waypoint. It is an inefficient use of the engines to continually adjust flight speed to try to narrow the ‘dead band’ of 1 minute. As the aircraft approaches the target waypoint, it becomes more important to track the RTA more closely, and equally the degree of predictability increases. Thus the value of DT can be reduced. For example, DT may be 5 seconds when the aircraft is 10 NM from the waypoint.